1. Field of the Invention
THIS INVENTION relates to aircraft aircrew life support systems and is particularly concerned with integrated breathing demand regulator and garment inflation pressure control systems.
2. Description of the Prior Art
The enhanced agility of modern high performance aircraft designs give such aircraft the ability to perform very highly accelerative manoeuvres both at low altitude and at high altitudes, e.g. in excess of 12000 meters (40000 ft). To take advantage of this agility an aircrew member flying the aircraft must be protected against G-induced loss of consciousness, known as G-loc, as well as the effect of exposure to high altitude in the event of loss of cabin pressure. In this regard, unless otherwise specified, references to altitude are to be understood as references to the altitude equivalent to the pressure within an enclosure or cabin within which an aircrew member is situated and which is usually pressurised in relation to the external ambient pressure with the consequence that "cabin altitude" is related to but usually less than the actual altitude of the aircraft.
The partial pressure of oxygen in air decreases with increasing altitude (decreasing total pressure) so that the concentration of oxygen in breathing gas supplied to the aircraft aircrew member must be increased with increasing cabin altitude to maintain the oxygen partial pressure above the minimum value necessary for it to be able to diffuse through the lung tissue and pass to the haemoglobin or red corpuscles in the blood. If, at aircraft operating altitudes above 12000 meters, there is total or partial loss of cabin pressure which causes cabin pressure to fall below 12000 meters equivalent pressure then the overall pressure of the breathing gas delivered to the aircrew member must be increased to a value above cabin ambient pressure so that the minima critical oxygen pressure is maintained in the lungs, this being referred to as positive pressure breathing (PPB).
Positive pressure breathing at high altitude is aided by exerting pressure around the chest to give support and to assist the aircrew member in exhaling used gas from his lungs against the positive pressure in his breathing mask and to enable breathing to be sustained until the aircraft has descended to 12000 meters or below. To meet this requirement the aircrew member wears an inflatable counter-pressure garment ("jerkin") around his chest and back area which is inflated to the same pressure as the pressure in the breathing mask during positive pressure breathing, conveniently by being connected for inflation by breathing gas delivered to the breathing mask.
To counter the effects of high G-load the aircrew member wears an inflatable G-protection trouser garment ("G-suit") which is inflated from a source of high pressure gas, such as engine bleed air. Inflation of the trouser garment may be in response to signals from one or more accelerometers located in the aircraft for sensing accelerative forces, or in response to movement of an inertia mass provided as part of an inflation control valve assembly. When inflated, the trouser garment restricts the flow of blood into the lower extremities of the body where it tends to be forced under the action of the G-load to which the aircrew member is subjected.
It has been found that protection against G-loc is further enhanced by providing positive pressure breathing during periods when high G-loads are being experienced. The increase in breathing pressure causes an approximately equal increase in heart level blood pressure thereby increasing the flow of blood to the brain.
On exposure to altitudes which demand positive pressure breathing it is advantageous to inflate the trouser garment to a pressure three to four times that of the pressure in the breathing mask even at times when aircraft flight manoeuvres are not such as to give rise to high G-load. This inflation of the trouser garment counteracts the tendency for blood to be forced into the lower extremities of the body by the high pressure in the lungs and by the counter-pressure garment, which reduces the circulation of blood from the heart to the brain. However, when both altitude and G-load conditions give rise to a requirement for positive pressure breathing, the trouser garment should be inflated to a pressure appropriate to the higher of the prevailing G-load or altitude signals.
It is common practice now to provide oxygen-enriched air as breathing gas for an aircrew member of a high performance aircraft from an on-board oxygen generating system (OBOGS) which includes molecular sieve beds comprising zeolite material suited to the retention of nitrogen whilst permitting oxygen to pass through the beds.
A problem with respect to demand valve operation in a breathing regulator suitable for accommodating the lower range of breathing gas pressure available from an OBOGS is overcome by a breathing regulator disclosed in EP-A-0,263,677 (Normalair-Garrett) which provides positive pressure breathing when the cabin altitude exceeds 12000 meters and, also, when high G-loads are being experienced. Above 12000 meters cabin altitude, an aneroid valve expands to increasingly restrict the flow of gas from a breathing-pressure control chamber so that pressure in this control chamber increases thereby increasing the pressure of the breathing gas at the regulator outlet to which both breathing mask and counter-pressure garment or jerkin are connected.
When the aircrew member is subjected to high G-loads, i.e. between 3.5 G and 9 G a further valve regulating outflow from the breathing-pressure control chamber is signalled pneumatically by an anti-G valve to move towards increasingly restricting outflow of gas from the breathing-pressure control chamber so that pressure in that chamber increases to provide (increased) positive pressure breathing in the event that the cabin altitude is below that at which the same degree of positive pressure breathing would be provided. The anti-G valve is an electro-pneumo-mechanical device that controls a supply of inflation air to the G-suit in accordance with sensed G-loads and the signal to the further valve of the demand regulator is obtained by tapping the inflation air line from the anti-G valve to the G-suit.
Further disclosures of aircraft aircrew life support systems and apparatus which control inflation of a G-suit worn by the aircrew member and regulate delivery of breathing gas in accordance with the breathing demand of the aircrew member are to be found in U.S. Pat. No. 4,230,097 (Intertechnique), U.S. Pat. No. 4,638,791 (Boeing) and GB-A-2,051,417 (Intertechnique), this last disclosing a unitary or integrated breathing demand regulator and G-suit inflation control valve in which, however, the demand regulator and the control valve are functionally separate.
These prior art systems (other than EP-A-0,263,677) treat breathing gas and jerkin pressure requirements and G-suit inflation pressure requirements as separate functions to be provided by individual sub-systems integrated, functionally, only to the extent of sharing input data (such as anticipated and/or realised G-loads) output from a common source.
A system having greater functional integration of such sub-systems to provide better control and coordination of their respective functions and, especially to provide optimised responses to abrupt change in aircraft flight conditions, is disclosed in EP-A-0 419 183 (Normalair Garrett).
This system comprises G-suit inflation control means and breathing demand regulator means disposed in a common housing. The G-suit inflation control means comprises a spool valve which is moved in an opening direction by a pneumatic actuator to allow inflation gas to flow to the G-suit, and a diaphragm mounted vent valve biased by a spring towards opening a vent port for deflation of the G-suit. An electronically controlled torque motor controls a valve member for regulation of a servo-pressure which acts to cause the pneumatic actuator to move the spool valve to an open position and to move the vent valve to close the vent port. The torque motor is signalled by a self-contained electronic control unit which receives and processes acceleration signals and cabin pressure signals. The control loop is closed by a feedback signal from a pressure transducer which senses G-suit inflation pressure.
Whilst certain advantages are offered by use of a self-contained electronic control unit dedicated to control of G-suit inflation, there are penalties in terms of cost, space and weight, because the design of the unit is complicated by the need to perform scheduling tasks.
A solution in an aircraft having an on-board utilities system computer is to use that computer to perform the control functions for the G-suit inflation control means. Whilst this would totally replace the dedicated electronic control unit and, possibly, provide the smallest, lightest package, a major disadvantage is that valve performance is directly limited by the update rate of the computer. Also, it may be considered inappropriate to use the computer to perform a computationally intensive control task which could more easily be satisfied by dedicated analogue electronics.
There is now a requirement in some high performance aircraft for the G-suit to be inflated to the higher of pressure requirements for protection against G-load and exposure to altitude above 12000 meters as might occur if the aircraft suffered a cabin decompression above such altitude when flying a highly accelerative manoeuvre. It is a further requirement that the G-suit be inflated to a pressure of three to four times breathing gas pressure in the event that the aircrew member has to eject from the aircraft above 12000 meters altitude.
Whilst the first mentioned requirement can be met by suitable software in the electronic control unit of the system disclosed in EP-A-0 419 183, there is no provision for continued functioning of the unit and the G-suit inflation control means when electrical power is lost following ejection from the aircraft, to give protection against exposure to altitude.